Final Cut Pro 5.1 UB- problem opening old project files

I have FCPro 5.1.2 UBinary running just fine, but I have troubles opening old projects from FCPro 3.0.9 for OS9: a dialogbox says “the project is unreadable or coming from a more recent version of FinalCutPro”!!
There’s no version following 5.1.2 UB, anyway…

It happens on a good half of my V3 projects and they open peacefully on the original FCPro 3 they were done with.
I cannot find nothing useful on the apple KB, any clue?